AITA for not letting my miracle baby niece be my flower girl at my wedding?

I always wanted to be a bride. Since I was little, I know exactly what my wedding will be like: Venetian masquerade themed, at our family's centuries-old winery, with my toes dyed black and wearing a Victorian era-inspired dress with a sullen expression. I've worked since I was 16 to save up for this. I won't be taking my husband's last name, but I will change my middle name to match his: Quincy. It took me multiple tries in high school to memorize a reading of Shakespeare's sonnet 18 but with my voice and facial expressions changed to convey a melancholy, almost sarcastic tone. I even have the entire honeymoon planned out on a Google Map. I want this to be the wedding of a lifetime, to make it an unforgettable experience for our guests that they'll be talking about for years.<br><br>My brother and his wife "baby-trapped" him and had a baby girl almost 4 months ago, naming her Ophelia "Ofie" Elara. This was a surprise to everyone, especially their 12-year-old daughter who was the intended "baby" in the house. Ofie was an emergency cesarean section, at 28 weeks my sister-in-law had a placental abruption that almost took both of their lives. I'm not good with babies; I'm a nurturer, but I'm more of a plant mom than a baby mom. I'm happy to do my part as aunt and watch Ofie now and then, but I just don't connect with her as much as I thought I'd connect with my niece or nephew.<br><br>I've been in the wedding party for multiple weddings, and I know how a badly-behaved flower girl or ring bearer can bring the whole ceremony down. I wanted someone more...predictable. My 12-year-old cousin, who I've gotten to know really well in the last few years since the pandemic started. She's calm, quiet, and doesn't fidget much. She agreed, and I bought her the dress I wanted her to wear. Her parents loved it too, and said if I'd pay for the hair and makeup I'd be paying for the other bridesmaids, they'd get her travel and hotel stay.<br><br>That was in August, and I was planning for June of next year. Well, in September my brother and his family came to visit, and I showed Ofie her "big girl room" in case she ever spent the night when she was older (it was basically an investment in my future, as I have no plans on moving unless I have to for work). That's when my SIL casually mentioned that Ofie could be my flower girl, and I said that I already had my flower girl picked out (Cousin Maura). My SIL said Ofie would be such a beautiful flower girl, and I said, "I know, that's why I picked Maura." My BIL was there during our conversation, but he didn't say a word. I thought that was the end of it.<br><br>Fast forward to December when we're visiting their home in another state. We're sitting around their dining room table looking at photos of my wedding dress (it's being made by the same company as her wedding dress) when my SIL "jokingly" brings up Ofie being my flower girl. This time, my BIL speaks up and says Ofie is "such a sweet beautiful baby" and that it would be "good practice" for her to be a flower girl at my wedding. I said she'd be an imperfect flower girl, and that she might ruin my wedding walking down the aisle. My BIL said that was an "incredibly-assholish" thing for me to say, and that I "should be grateful that Ofie is still alive." I said, "I don't want a baby as my flower girl." He said I "didn't have a choice" and that Ofie would be my flower girl. He then said I was being selfish and "self-absorbed" for not letting Ofie be flower girl, especially since she's my only niece and I don't know "if I'll ever have children." I said, "Sorry, I guess Ofie won't be my flower girl." He said, "Sorry, but she will."<br><br>I left their dining room and went to my bedroom, where my fiancé was waiting. We packed our bags and said our "goodbyes" and left. My brother texted, asking if I could "get over myself" and let Ofie be flower girl. I said Maura has already agreed, Ofie is too young to remember it, and that I wasn't going to let him strong-arm me into letting his baby be flower girl. My mom is saying I need to let Ofie be the flower girl, "as a tribute to how strong and brave she is." I said no. My friends are all on my side, but everyone else says Ofie would be an adorable flower girl and that I'm being petty for not letting her be.